The hydraulic rotary rig set up on the south bank of the River Dee isn't just drilling; it's extracting continuous Shelby tube samples from the organic silts that underlie much of Aberdeen's former harbour margins. When you're planning a tunnel through the city's complex glacial and post-glacial deposits, standard penetration testing won't give you the full picture. Our team deploys thin-walled sampling and in-situ vane shear tests to capture the undrained shear strength of the soft, often laminated clays found beneath Union Street and the Merchant Quarter. We combine this with triaxial testing to define the critical state parameters for these normally consolidated soils, and we use in-situ permeability testing to understand groundwater flow through the granular lenses trapped within the till. The goal isn't just a report; it's a ground model that lets your TBM operator know exactly what to expect metre by metre, especially where the weathered granite bedrock rises unpredictably close to the tunnel crown near the Holburn Street area.
Predicting settlement in Aberdeen's mixed-face conditions isn't about textbook formulas; it's about understanding how a TBM interacts with weathered granite one minute and soft organic silt the next.
Our approach and scope
Site-specific factors
The classic mistake we see in Aberdeen is treating the entire tunnel alignment as if it's in competent glacial till, just because the borehole logs from the feasibility stage looked reasonable at 50-metre centres. The reality is that the till contains lenses and laminae of soft clay and saturated sand that can trigger a rapid, uncontrolled loss of face if the TBM's closed-mode support pressure isn't calibrated to the local pore pressure regime. We've reviewed projects where a single, unmapped buried channel of the River Dee's palaeo-course — filled with very loose, saturated alluvium — caused a surface settlement trough that cracked Victorian masonry buildings 300 metres from the shaft. Proper ground investigation for soft ground tunnels in this city isn't a box-ticking exercise for your CDM designer; it demands a phased approach, starting with a desktop study of the BGS superficial deposit maps, followed by targeted cone penetration tests and instrumented boreholes that capture the spatial variability of the material. The cost of this investigation is orders of magnitude less than a single day of TBM downtime or a compensation claim from Network Rail.
Regulatory framework
BS EN 1997-2:2007 (Eurocode 7 – Ground investigation and testing), BS 5930:2015+A1:2020 (Code of practice for ground investigations), BS 1377 (Methods of test for soils for civil engineering purposes), CIRIA C760 (Guidance on embedded retaining wall design), ITIG (International Tunnelling Insurance Group) Code of Practice for Risk Management
Complementary services
Tunnel Face Stability Assessment
We determine the required TBM support pressure using undrained shear strength profiles from in-situ vane tests and CIU triaxial data, combined with pore pressure monitoring in the tidally influenced superficial aquifers common along Aberdeen's coastal margin.
Settlement Prediction & Building Damage Assessment
Using oedometer consolidation parameters and non-linear stiffness degradation curves from triaxial testing, we predict short and long-term settlement troughs. We assess the tensile strain risk to adjacent masonry and reinforced concrete structures following the CIRIA C760 methodology.
Mixed-Face Ground Characterisation
We map the weathered granite bedrock profile using rotary-cored boreholes and integrate this with CPTu data to delineate the rockhead. This defines the length of tunnel in mixed-face conditions, allowing you to plan for cutterhead wear and face instability at the transition zones.
Groundwater Control & Dewatering Advice
Falling-head and constant-head permeability tests in standpipe piezometers quantify the hydraulic conductivity of the granular lenses within the till. We provide the input parameters for numerical groundwater models that simulate the effects of pumping on nearby wells and surface water features like the River Dee.
Typical parameters
Common questions
What specific geotechnical risks do Aberdeen's glacial deposits pose to a soft ground TBM?
The main risk is the abrupt lateral and vertical transition from stiff, overconsolidated lodgement till into soft, normally consolidated post-glacial silts and clays. These soft pockets require higher face support pressure, and if they contain granular lenses, the tidal influence from the North Sea can cause sudden pore pressure drops, destabilising the face. A phased investigation with closely spaced CPTs is the only reliable way to map these transitions.
How do you determine the soil parameters needed for a PLAXIS 2D or 3D tunnel analysis?
We carry out a suite of triaxial tests — CIU for undrained strength and CID for effective stress stiffness at in-situ density. Oedometer tests give us the compression and swelling indices for the Soft Soil model, while permeability tests provide the hydraulic conductivity for coupled consolidation analysis. We don't rely on generic correlations; we use site-specific data from the Aberdeen formation.
What is the typical cost for a site investigation focused on a short utility tunnel in Aberdeen?
For a targeted investigation with 3 to 4 boreholes, in-situ testing, and a laboratory schedule covering classification and triaxial strength, budgets typically fall between £3,780 and £15,040. The final figure depends on access constraints in the city centre, the depth of the weathered granite, and the number of specialist tests required to characterise the soft soil units.
Can you investigate the ground beneath Aberdeen's listed buildings without causing damage?
Yes, we use low-headroom drilling rigs and, where vibration is a concern, we deploy CPT rigs that can operate in tight back gardens or warehouse basements. For sensitive structures, we can combine surface geophysics with targeted window sampling to minimise the physical footprint while still getting the soil stiffness data your tunnel design needs.